Before Sita harana, Ram exchanged original Sita from Agni. And no one knows it. Anyway Ravan could not harm a duplicate one then what would happen if original Sita were not exchanged? why the original Sita was not there?
Why Rama used a duplicate Sita in Ramayan?
As per Valmiki Ramayana this exchange story might not be true.
As per Skanda Purana also there is no concept of Maya or Chaya or Duplicate Sita but Vedavati:
Why Ravan could not harm a duplicate one?
Due to a curse by Nalkubera, Ravana couldn't harm any woman's chastity after he forcefully harmed Nalkubera's would be wife's chastity. So, there is no point being a duplicate or original Sita.
Before that curse he used to harm, even he tried to harm chastity of Vedavati (in previous birth whom you are referring as Duplicate Sita as per Skanda Purana) but she entered in fire. Ravana himself accepted this:
शप्तोऽहम् वेदवत्या च यदा सा धर्षिता पुरा || ६-६०-१०
सेयम् सीता महाभागा जाता जनकनन्दिनी |
Further, I (Ravana) was formerly cursed by Vedavati when she was humiliated by me. The same Vedavati is born as the highly fortunate Sita, the daughter of Janaka.
Why the original Sita was not there? What would happen if original Sita were not exchanged?
As per Valmiki Ramayan original Sita was there, nothing happened because
इमामपि विशालाक्षीं रक्षितां स्वेन तेजसा | रावणो नातिवर्तेत वेल मिव महोदधिः || 6.118.16
Rama: Ravana could not violate this wide-eyed woman, protected as she was by her own splendour, any more than an ocean would transgress its bounds.
न च शक्तः सुदुष्टत्मा मनसापि हि मैथिलीम् | प्रधर्षयितुमप्राप्यां दीप्तामग्निशिखामिव || 6.118.18
Rama: The evil-minded Ravana was not able to lay his violent hands, even in thought, o the unobtainable Seetha, who was blazing like a flaming tongue of fire.
Then, why Rama asked for Agni pariksha?
बालिशो बत कामात्म रामो दशरथात्मजः | इति वक्ष्यति मां लोको जानकीमविशोध्य हि || 6.118.14
Rama: The world would chatter against me, saying that Rama, the son of Dasaratha, was really foolish and that his mind was dominated by lust, if I accept Seetha without examining her with regard to her chastity.
Note: Many stories from Skanda Purana and Valmiki Ramayana are different, it might be possible that they tell the story of Rama from different different Yuga.